I imagine it’s a lot of fear and anxiety but why does the body react so dramatically?
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some people have an abnormally strong ‘vasovagal response’. scientists actually don’t know exactly how this works. When these people see blood or get suddenly stressed, they have a ‘syncope’ (sin-ko-pi). Their heart suddenly beats slower, so less blood is getting pushed around the body for a short period. The blood vessels also open up, lowering pressure all over the body, so less of the blood can get to the head. Then the brain can’t get enough oxygen and the person passes out.
Fainting like this is pretty common and doesn’t cause any real damage, unless the person faints while standing and hits their head. So—if you ever get surprised or cut yourself and feel woozy all of a sudden, SIT DOWN RIGHT AWAY. This is why we don’t give blood standing up.
The vagus nerve runs from the brain to the heart and gut. It’s such a big nerve that it does stuff we used to think the brain does like keeping your heart ticking and your guts moving. Some scientists explain the vasovagal response as coming from a very old (in terms of evolution) reaction that kept some humans from bleeding out when they got cut. But I stress that the vasovagal response is really mysterious, as no-one understands the link between a conscious perception (ah! blood!) and unconscious physiological regulation.
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